1. Field of the Invention
The invention concerns an electrostatic method and apparatus for painting small elongate objects which extend along a principal axis, in which the objects are passed along a transporter forming a loop which is almost closed, while rotating around their respective axes, a rotating bowl sprayer being disposed substantially on the axis of the loop so as to direct paint onto the objects.
2. Description of the Prior Art
Rotating bowl electrostatic sprayers direct a thin sheet of atomized paint, contained between two conical surfaces with almost the same angle at the apex, from the edge of a component rotating at high speed, onto the surface of which the paint is deposited. The distance separating the sprayer from the objects to be painted is of the order of a few decimeters.
In painting objects with small dimensions, that is to say with dimensions less than the distance of the objects from the sprayer, it will be clear that the cost-effectiveness of the operation is dependent on the objects being presented while moving in such a way as to intercept a substantial portion of the sheet of atomized paint. Moreover, if the objects extend along a principal axis, it is preferable to rotate them around this axis so as to obtain a regular thickness over their periphery, and to present this principal axis substantially perpendicular to the sheet of paint so that the components holding the objects are protected from the paint. The solution generally adopted consists in the use of a transporter forming a loop, which is almost closed, around a horizontal axis. On this the objects are mounted with their principal axis parallel to the axis of the loop and so as to rotate several times on their own axes while moving around the loop. The sprayer is disposed with the bowl on the axis of the loop so that the distance to the objects is constant. The objects move around the sprayer on a part-cylindrical surface delimited by two circles coaxial with the loop, traced out by respective ends of the principal axes of the objects. The axis of rotation of the sprayer bowl is at an angle to the axis of the loop, however, so that the area in which the part-cylindrical surface defined by the axes of the objects and the sheet of atomized paint is tangential to the two circles delimiting the part-cylindrical surface over which the objects move. Thus as they move around the loop the objects may be coated with paint over their entire length. The rotation of the objects around their axes and the electrostatic force resulting from the action on the electrically charged paint particles of an electrostatic field created by a difference in potential between the rotating bowl and the objects ensures a regular deposit of paint on the periphery of the objects (as seen along the axis). However, the inclined disposition of the axis of rotation of the bowl results in excess thickness at the ends of the objects, since the rate at which the aforementioned intersection area moves along each object becomes zero at the ends, at which it is tangential to the delimiting circle, and is maximum in the center.
In practice there is no improvement in this when the sprayer is equipped with an annular nozzle behind the bowl, supplied with a gas under pressure and oriented so that the gas jets are directed onto the initial part of the sheet of paint. This sheet then assumes a generally conical shape, thickening slightly under the effect of turbulence. This arrangement, sometimes referred to as a "shower" spray, enables the sprayer to be moved back relative to the center of the part-cylindrical surface. The differences in thickness of the paint in the longitudinal direction are not reduced by this measure, however, as in addition to the excess thicknesses at the ends there is then a lack of symmetry relative to the center of the object.
Some objects may be of a shape such that a variation of the quantity of paint deposited along the length of the object is desirable, but it is unfortunately rare for the required variation to correspond to the variation resulting from the oblique disposition of the rotation axis.
Because of the small dimensions of the objects, the period during which they are coated with paint is of limited duration. If it were required to modulate the quantities of paint sprayed by controlled displacement of the sprayer, as is common practice for coating certain large objects, it would be necessary for the period of such displacement to be short relative to the total coating duration, so as to affect all objects uniformly. The corresponding control system would be complex, expensive and difficult to adjust.
An objective of the invention is to provide an electrostatic method for painting small objects in such a way that the longitudinal distribution of paint is regular.
Another objective of the invention is to provide an electrostatic method for painting small objects wherein the longitudinal distribution of paint may be modulated in a relatively simple manner.